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Tornadoes – how frequently do they hit New Zealand?

Tornadoes – how frequently do they hit New Zealand?

Tornadoes, like the one that hit Auckland’s western suburbs today, are relatively rare events in New Zealand.

On average there are around seven moderate to strong tornado events reported in New Zealand each year.

NIWA meteorologist, Dr Richard Turner, says “fatalities associated with tornadoes are rare in New Zealand, with the most recent cases being the Albany tornado last year, which killed 1 person, and a tornado near Waitara in August 2004 which killed 2 people.”

“Auckland is hit by a tornado on average less than once per year, but there is considerable variability from year to year with some years getting none,” says Dr Turner.

New Zealand tornadoes are neither as common nor as destructive as those that occur over the plains of the United States, but even small scale tornadoes, like today’s event can cause damage, injury and even deaths. The west coast of the South Island and the North Island coast from Taranaki to Northland have been particularly affected by tornadoes in the past.

Tornadoes in New Zealand are usually around a few tens of metres wide and have tracks of just a couple of kilometres. They are extremely localised and the damage is very confined to the actual tornado itself.

A tornado will typically last for a few minutes, track across the land for 2 to 5 kilometres and will have a diameter of 20 to 100 metres. Wind speeds are in the order of 115 to 180 km/h. At the more extreme end, some tornadoes track for over 100 kilometres, are over 1 kilometer wide and have winds up to 480 km/h – such tornadoes are extremely rare, anywhere in the world.

Dr Turner says “In New Zealand most tornadoes are associated with pre-frontal squall lines - bands of thunderstorms embedded in strong unstable pre-frontal northwesterly flow.”

“The thunderstorms have very strong updrafts and if these occur in an environment in which the wind directions rotate as the air rises, the updraft can start to spin and a mesocyclone can form. It is from these mesocylcones, that can be as little as 1-2 kilometres across, that tornadoes are spawned.”

NIWA maintains a catalogue of major weather events in New Zealand over the last 200 years called the New Zealand Historic Weather Events Catalogue.

The information has been collated from newspaper reports, journals, books and databases kindly provided by various organisations and individuals. For each event we identify the regions affected, the hazards types associated with the event and the resulting impacts.

The most damaging and lethal tornado in New Zealand occurred at Frankton (Hamilton) on 25 August, 1948. The tornado carved a 100–200 m swath through the suburb, causing 3 deaths, 12 injuries, damaging 150 houses and 50 businesses with an overall damage cost of $60 million.

The most recent events occurred on the west coast of the North Island, when a swarm of at least 12 tornadoes hit the Taranaki Coast on Wednesday the 4th and Thursday the 5th of July 2007 causing widespread damage in the region.

Oakura, a town 12 km southwest of New Plymouth was most affected. Roughly 50 houses suffered major damage, some of it irreparable, when two tornadoes ripped through the town.

Last year, a tornado in the Auckland suburb of Albany killed one person, sent cars airborne and did about $10 million in damage over a 15 km path.

Many other tornadoes in remote rural areas will be unreported.

Current research at NIWA is investigating very high resolution weather forecast models (which demand huge computational resources) ability to predict the occurrence of mesocyclones. If successful, this could potentially aid forecasters in their ability to identify mesocyclone formation between 6 and 12 hours in advance, increasing warning times.

ENDS

More information:

What are the warning signs of a tornado?
• Hail or heavy rain followed by dead calm or a fast, intense wind shift.
• Hail stone size can indicate storm intensity.
• Load continuous roar or rumble, much like the sound of an approaching freight train.
• At night, small, bright, blue-green to white flashes at ground level near a thunderstorm.
• A large, dark, low-lying cloud.
• Dark, often greenish sky.
• Cloud of debris. An approaching cloud of debris can mark the location of a tornado even if a funnel is not visible.

For more information go to: http://www.niwa.co.nz/natural-hazards/hazards/extreme-weather-winds-and-tornadoes

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